High CO₂ in the mantle source of ocean island basanites

Some of the most CO2-rich magmas on Earth are erupted by intraplate ocean island volcanoes. Here, we characterise olivine-hosted melt inclusions from recent (<10 ky) basanitic tephra erupted by Fogo, the only active volcano of the Cape Verde Archipelago in the eastern Atlantic Ocean. We determine H2O, S, Cl, F in glassy melt inclusions and recalculate the total (glass + shrinkage bubbles) CO2 budget by three independent methodologies. We find that the Fogo parental basanite, entrapped as melt inclusion in forsterite-rich (Fo80-85) olivines, contains up to -2.1 wt% CO2, 3-47 % of which is partitioned in the shrinkage bubbles. This CO2 content is among the highest ever measured in melt inclusions in OIBs. In combination with -2 wt% H2O content, our data constrain an entrapment pressure range for the most CO2-rich melt inclusion of 648-1430 MPa, with a most conservative estimate at 773-1020 MPa. Our results therefore suggest the parental Fogo melt is stored in the lithospheric mantle at minimum depths of -27 to -36 km, and then injected into a vertically stacked magma ponding system. Overall, our results corroborate previous indications for a CO2-rich nature of alkaline ocean island volcanism. We propose that the Fogo basanitic melt forms by low degrees of melting (F = 0.06-0.07) of a carbonenriched mantle source, containing up to 355-414 ppm C. If global OIB melts are dominantly as carbon-rich as our Fogo results suggest, then OIB volcanism may cumulatively outgas as high as -16-21 Tg of carbon yearly, hence substantially contributing to the global deep carbon cycle.